The transesterification process in which a polycarbonate is prepared by reacting a diester carbonate with a dihydroxyaryl compound, has rather simple preparation steps, and is superior to the phosgene method with respect to operation and cost. Further, the transesterification process has recently been appreciated from an environmental aspect since phosgene and methylene chloride, which have strong toxicity, are not used.
However, the transesterification process has not been adopted in a large scale industrial process. The greatest reason for this is that the physical properties, such as color, stability to hydrolysis and heat yellowing resistance of polycarbonate, prepared by using the conventional transesterification process are inferior to the properties of polycarbonate prepared by the phosgene process. There are several reasons why these physical properties are not good. One of the reasons is the influence of the material of the reactors.
To solve the problem, for example, U.S. Pat. No. 4,383,092 describes a method for inhibiting color development in a polymer by using a special material of non-ferrous or non-stainless steel materials, such as tantalum, chrome and nickel for the material of the reactor. However, these metals are expensive, and do not easily work, so the problem with the method in U.S. Pat. No. 4,383,092 is that the reactors are very expensive to make. Further, in U.S. Pat. No. 4,383,092, there is no description of the physical properties, such as stability to hydrolysis and heat yellowing resistance, etc., of the resulting polymer.
JP-A-4-72327 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") describes a method in which a metal material having a copper and/or nickel content of 85% by weight or more is used as the surface which contacts the reaction mixture. However, it has a problem that the reactors become expensive. Also, there is no description of the physical properties, such as stability to hydrolysis and heat yellowing resistance, etc., of the resulting polymer.
Further, there are descriptions of a method in which a material having a nickel and/or aluminum content of 60% by weight or more is used as the material of the reactor (JP-A-5-125168), a method in which a material having a nickel and/or molybdenum content of 60% by weight or more is used as the material of the reactor (JP-A-5-125169), a method in which a material having a nickel and/or carbon content of 60% by weight or more is used as the material of the reactor (JP-A-5-125170), a method in which a material having a nickel and/or chrome content of 60% by weight or more is used as the material of the reactor (JP-A-5-125172), a method in which a material having a copper and/or aluminum content of 60% by weight or more is used as the material of the reactor (JP-A-5-125173), a method in which a material having a copper and/or zinc content of 60% by weight or more is used as the material of the reactor (JP-A-5-125174).
Further, JP-A-4-332725 describes a method in which a material is used where the contact area with the reaction liquid in a reactor is metal plated, and a stainless steel reactor is used in which electrolytic polishing is performed on the contact area with the reaction liquid.
Although several materials are described as the material for a reactor as mentioned above, they are special materials, the reactor itself is expensive, and it is inconvenient for an industrial process. Conventionally, stainless steel reactors are understood to cause color development of the product polymer, but it is difficult to obtain a polymer having a high molecular weight and good physical properties. Further, there is no material in which both the color and the physical properties are successfully and simultaneously provided.
An object of the present invention is to provide a reactor in which a polycarbonate having excellent color, stability to hydrolysis and heat yellowing resistance can be prepared, even when a stainless steel reactor is being used.